Method and apparatus for producing a post-mixed, stabilized scarfing pre-heating flame

ABSTRACT

A process for thermochemically scarfing a metal workpiece comprising:

BACKGROUND

This invention relates to the thermochemical desurfacing of metal workpieces, commonly called scarfing. More specifically, this invention comprises a method and apparatus for preheating the surface of a metal workpiece where a scarfing reaction is to be started.

A complete scarfing cycle usually consists of three steps: (1) positioning the workpiece in register with the scarfing units, (2) preheating the workpiece to form a molten puddle, and (3) carrying out the scarfing reaction with a stream of scarfing oxygen while causing relative motion between the workpiece and the scarfing unit or units. This invention is concerned principally with the preheating step.

The prior art discloses several methods for performing the preheating step. Jones et al, in U.S. Pat. No. 2,267,405, discloses preheating with a flame produced by combining oxygen and fuel gas within a torch and igniting the gas mixture as it leaves the torch. The problem with combining oxygen and fuel gas within a torch, hereinafter referred to as "premixing", is that the explosive mixture is subject to flashback, i.e. ignition inside the torch, which can damage the torch and become a safety hazard.

An improvement in the pre-mixed flame was disclosed by Jones et al in U.S. Pat. No. 2,356,197, in which oxygen and fuel gas are combined just prior to being discharge from the nozzle. While this was an improvement in the state of the art, the apparatus was still subject to flashback. If the outer nozzle were to be plugged, e.g. with spattered metal, while the oxygen and fuel gas holes inside the unit remained open, the two gases could mix inside the unit, thereby creating an explosive mixture subject to flashback.

Allmang's U.S. Pat. No. 3,231,431 discloses postmixed preheating apparatus wherein the oxygen and fuel gas are combined outside the unit, thereby completely eliminating the possibility of flashback. However, the intensity of the flame produced by this post-mixed apparatus is limited. While Allmang's method can be used to preheat hot workpieces, its low intensity flame requires an unacceptably long time to preheat cold workpieces.

Lytle's U.S. Pat. No. 3,752,460 discloses post-mixed preheating apparatus that uses a stream of "trap" oxygen to decrease preheating time. While Lytle's invention is an improvement over Allmang, Lytle's apparatus is not capable of preheating relatively cold workpieces fast enough for commercial operations.

Engel's U.S. Pat. No. 3,966,503 discloses a method for making an instantaneous scarfing start, that reduces the time required for preheating the workpiece virtually to zero. Engel's method is faster than the method of the present invention; however, Engel's method requires a rod feed mechanism and a high intensity jet of oxygen, not required by the present invention. Hence, the present invention is advantageous when an instantaneous scarfing start is not required, but a fast start on cold steel is desired.

Until the present invention, it has not been possible to rapidly preheat a portion of the surface of a relatively cold metal workpiece to scarfing temperature, using a flame, without danger of flashback, or without using rods, high intensity blowpipes or other adjuvant material.

OBJECTS

Accordingly, it is an object of this invention to provide a method as well as apparatus for scarfing the surface of a workpiece that provides acceptably short preheat times for scarfing relatively cold workpieces, without being subject to flashback, and without requiring adjuvant material.

It is another object of the present invention to provide a method as well as apparatus for producing a post-mixed scarfing preheating flame that is more intense than those produced by the prior art.

SUMMARY OF THE INVENTION

The above the other objects, which will readily be apparent to those skilled in the art, are achieved by the present invention, one aspect of which comprises:

A process for thermochemically scarfing a metal workpiece comprising:

(1) preheating a spot on the surface of the workpiece where the scarfing reaction is to begin by directing a post-mixed preheating flame at said spot, said preheating flame being formed by:

(a) discharging at least one stream of preheat oxidizing gas and at least one stream of preheat fuel gas from separate ports in such a manner that said streams impinge external to their discharge ports, above the work surface and in such manner that the axes of said streams form an acute included angle between them, and

(b) stabilizing said preheating flame by discharging a low-intensity stream of oxidizing gas, the direction of said stabilizing stream being in the same general direction as the direction of said flame and proximate to the impingement of said preheat oxidizing gas and preheat fuel gas streams, and

(c) continuing steps (a) and (b) until said spot reaches its oxidizing gas ignition temperature, and thereafter

(2) directing a stream of scarfing oxidizing gas at an acute angle to the work surface at said preheated spot, while simultaneously

(3) causing relative movement between said scarfing oxidizing gas stream and said work surface, thereby producing a scarfing cut.

A second aspect of the invention comprises:

Scarfing apparatus comprising: (a) means for forming a post-mixed preheating flame, (b) means for discharging a stream of scarfing oxidizing gas through a scarfing nozzle, and (c) means for producing relative motion between the scarfing oxidizing gas and a workpiece, characterized in that said means for forming said preheating flame comprises:

(1) orifice means for discharging a stream of preheat fuel gas, the axis of said orifice means being directed toward the workpiece to be scarfed,

(2) orifice means for discharging a stream of preheat oxidizing gas, the axis of said orifice being directed to intersect the axis of said preheat fuel gas orifice means at an acute included angle, external to said orifices and above the surface of the workpiece, and

(3) means for discharging a low-intensity stream of stabilizing oxidizing gas, said means comprising an orifice, the axis of which is directed proximate to the intersection of the axes of said preheat fuel gas orifice and said preheat oxidizing gas orifice and directed in the same general direction as the resultant of said axes.

The preferred oxidizing gas is oxygen and the preferred included angle of impingement between the preheat gas streams is from 5° to 50°. The preferred embodiment uses the same orifice to discharge the preheat stabilizing oxygen stream as well as the scarfing oxygen stream.

The term "oxidizing gas" as used throughout the present specification and claims is used to mean a gas containing an oxidizing agent. The preferred oxidizing gas is commercially pure oxygen, and for simplicity the term "oxygen" is used hereafter throughout the specification. However, the invention may be practiced using oxidizing gases other than pure oxygen. For example, the oxidizing gas for scarfing and stabilizing can be oxygen having a purity as low as 99 percent, or lower. However, the results will be poorer with impure oxygen, especially with oxygen of less than 99 percent purity. The preheat oxidizing gas can contain as low as 21 percent oxygen, i.e. it can be air, but preheat times will increase with decreasing oxygen percentage in the preheat oxidizing gas stream.

The term "preheat" is used to mean bringing a portion of the surface of a workpiece to its oxidizing gas ignition temperature; that is, the temperature at which the workpiece will ignite when in an atmosphere of oxidizing gas.

IN THE DRAWINGS

FIG. 1 is a side view of a scarfing unit illustrating a preferred embodiment of the present invention.

FIG. 2 is a sectional view of FIG. 1 viewed along line 2--2.

FIG. 3 is an enlarged side view of FIG. 1 illustrating the key elements of the invention.

FIG. 4 illustrates the preferred location of the molten puddle with respect to the scarfing oxygen stream for scarfing starts on the flat portion of a work surface.

FIG. 5 illustrates a start on the end of a work surface.

FIG. 6 graphically compares the preheat time obtained by practice of the present invention with prior art methods of preheating the work surface.

FIG. 7 illustrates an embodiment of the invention in which the preheat streams are discharged from the lower preheat block of the scarfing apparatus.

FIG. 8 is a side view of apparatus having separate stabilizing oxygen and scarfing oxygen ports.

FIG. 9 is a front view of the apparatus of FIG. 8 viewed along the lines 9--9.

FIG. 10 is a front view of an alternate method of constructing the apparatus of FIG. 8.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1, 2 and 3 illustrate a preferred embodiment of the invention. A typical scarfing unit is comprised of an upper preheat block 1, a lower preheat block 2, a head 3, and a shoe 4. The blocks 2 and 3 are called preheat blocks because preheating flames are discharged from these blocks in conventional apparatus. However, in the apparatus illustrated in FIGS. 1, 2 and 3, only the flames discharged from the upper preheat block are used for preheating. A slot-like scarfing nozzle 16, from which a sheet-like stream of scarfing oxygen is discharged, is formed by the lower surface 20 of the upper preheat block 1 and the upper surface 21 of the lower preheat block 2. The lower preheat block 2 is provided with a row of fuel gas ports 19, communicating with conventional suitable gas passages (not shown). Oxygen and fuel gas are supplied to head 3 through pipes (not shown) and then to the respective gas passages by means well known in the art. The shoe 4 rides on the surface of the workpiece W during scarfing to keep the scarfing nozzle positioned a constant distance Z (FIG. 3) from the work surface. The scarfing reaction is carried out by impinging on a molten puddle a sheet-like stream of scarfing oxygen discharged from nozzle 16 at an acute angle to the work surface, while relative motion is caused to take place between the workpiece and the scarfing unit.

In accordance with the invention, the upper preheat block is provided with a row of preheat fuel gas ports 17 and a row of preheat oxygen ports 18, each of said ports communicating with supply passages (not shown) for fuel and oxygen, respectively. While the drawing shows the preheat oxygen ports 18 located above the preheat fuel gas ports 17, the reverse arrangement, although not preferred, will also work.

The apparatus functions as follows. Preheat oxygen streams 9 from ports 18 and preheat fuel gas streams 10 from ports 17 impinge forming a combustible mixture. The impingement appears as a point 30 in FIG. 3. Upon ignition, the combustible mixture forms a flame 14, having a low intensity zone 13 and a high intensity zone 12. It has been found that the high intensity zone 12 may be lengthened so that its tip 27 is just above the surface of the workpiece W, thereby producing a longer, more intense flame, by stabilizing the preheat flame by providing a low intensity stream of oxygen the passes proximate to the point of impingement 30 and in the same general direction as the flame 14. By passing the low intensity stream 15 "proximate to" the point of impingement is meant that the stream should pass close to the point of impingement 30, but not through it. While the term "point of impingement" has been used, it should be recognized that the term "locus of impingement" would be more accurate, since there are many intersecting streams, hence many points of impingement; and furthermore, since the streams have thickness, the intersections are areas rather than merely points. Hence, for brevity, the term "impingement" is used throughout the specification and claims to mean the locus of the areas of impingement of the preheat fuel gas and preheat oxidizing gas streams. The preferred source of the stabilizing oxygen stream 15 is scarfing nozzle 16. Conventional valve means (not shown) are provided for producing the low intensity stream of oxygen 15 (lower in intensity than a scarfing oxygen stream) through scarfing nozzle 16.

The stream 15 should be directed in the same general direction as the flame. That is, if stream 15 were resolved into two vector components, one parallel to and one perpendicular to the direction of the flame, the vector component parallel to the flame should point in the same direction as the flame. Preferably, the projections of the axes of stream 15 and flame 14 cross forming an acute included angle, as illustrated in FIGS. 1 and 3. It is also preferable that the axis of the stabilizing stream 15 be parallel to that of the preheat fuel gas stream, as also shown in FIGS. 1 and 3.

The preheat fuel gas and oxygen streams must impinge at an acute angle, i.e. at an angle greater than 0° but less than 90°. The preferred range is 5° to 50°, and the most preferred impingement angle is 15°.

The stabilizing oxygen stream 15 from nozzle 16 must be of low intensity, i.e. with a lower nozzle velocity than that of the preheat oxygen and fuel gas from nozzles 18 and 17. Preferably, the nozzle velocity of the stabilizing oxygen is about 10% of that of the preheat streams.

If the preheat flame were not stabilized as described above, the length of the high intensity zone (from the impingement 30 to tip 27) would be so short that the preheating step could not be performed in acceptably short times unless the stand-off distance Z were reduced. Reducing the stand-off distance Z to bring the tip of the high intensity zone of an unstabilized flame close to the workpiece would subject the scarfing unit to much more damage from spattered metal and slag than occurs at normal stand-off distances.

Flames produced by fuel gas from lower ports 19 mixing with oxygen from nozzle 16, are used to sustain the scarfing reaction. These flames are not necessary during preheat, but fuel gas should flow from ports 19 during preheat to prevent their plugging.

After the molten puddle forms at spot B, the valve means controlling oxygen flow from slot 16 are adjusted to increase the intensity of the oxygen flow from low intensity to scarfing intensity, and relative motion between the workpiece and the scarfing unit is started, thereby producing a scarfing cut on the surface of the workpiece. During the scarfing operation, the preheat flames formed by streams 9 and 10 are left on at a lower intensity then during preheat to help sustain the scarfing reaction. A baffle 28 positioned above preheat ports 17 and 18, is used to prevent blow-off of the low intensity flame during scarfing.

There are several design variables to be decided upon when fabricating the apparatus of the present invention, many of which are not independent of each other. For conventional scarfing apparatus, the following are usually fixed:

(1) G, the angle between the scarfing oxygen and surface of the workpiece,

(2) X, the height of nozzle 16,

(3) Z, the standoff distance,

(4) U, the width of the scarfing unit, (see FIG. 2),

(5) the type of fuel gas available,

(6) the type of oxidizing gas available.

For each set of values for the above parameters, there will be an operable range and a preferred value for the variables used in designing preheating apparatus in accordance with the invention.

The following two tables give examples of values that have been found satisfactory for practicing the invention. Table I lists a set of typical values of parameters for conventional scarfing equipment, known to produce good scarfing.

                  TABLE I                                                          ______________________________________                                         G, scarfing oxygen angle                                                                               35°                                             X, height of nozzle 16  5.6 mm                                                 Z, standoff distance    25 mm                                                  U, width of scarfing unit                                                                             270 mm                                                  fuel gas               natural gas                                             oxidizing gas          oxygen                                                  ______________________________________                                    

Table II gives the operable range and the preferred value of the variables found useful for practicing the invention when the fixed parameters are those shown in Table I.

                  TABLE II                                                         ______________________________________                                                           Preferred Approximate                                        Variable          Value     Operable Range                                     ______________________________________                                         Diameter of preheat fuel                                                       gas ports 17:     1.0 mm    0.7 - 1.7 mm                                       Preheat fuel gas flow rate                                                     per port:         1.7 SCMH  1 - 3.5 SCMH                                       Spacing of preheat fuel gas                                                    ports (dimension Y, in Fig. 3)                                                                   6.0 mm    3 - 16 mm                                          Diameter of preheat oxygen                                                     ports 18          1.6 mm    1 - 2.3 mm                                         Preheat oxygen flow rate per                                                   port:             3.7 SCMH  1.5 - 6 SCMH                                       Spacing of preheat oxygen gas                                                  ports (dimension Y, in Fig. 3)                                                                   6.0 mm    3 - 16 mm                                          Impingement angle between axes                                                 of preheat fuel ports and pre-                                                 heat oxygen gas ports (angle                                                   D, in Figure 3)   15°                                                                               5° - 50°                             Distance 26 between surface                                                    20 and preheat fuel gas                                                        ports 16          10 mm     3 - 15 mm                                          Angle between preheat oxygen                                                   port axis and workpiece                                                        (angle H, in Fig. 3)                                                                             50°                                                                               40° - 75°                            Distances 31 and 32 from                                                       impingement 30 to preheat                                                      ports             15 mm     3 - 22 mm                                          Distances 29 (see Fig. 2)                                                      between center lines of port                                                   17 and port 18    4 mm      1.5 - 6 mm                                         Stabilizing Oxygen flow rate                                                   from slot 16 during preheating,                                                per cm of slot width                                                                             6 SCMH    3 - 10 SCMH                                        ______________________________________                                    

The variables shown in Table II are dependent upon each other. Therefore, if any are made significantly different from the preferred value, the preferred value and operable range of other variables may change. Of course, if any of the fixed parameters of Table I are changed, the preferred and operable ranges of some of the variables in Table II may change. Those skilled in the art will recognize that an almost limitless number of combinations of values for Tables I and II will give satisfactory results.

The preferred shape of ports 17 and 18 is circular, but other shapes will work. For example, the ports could be square or rectangular. A single elongated preheat oxygen nozzle could be used with a single elongated preheat fuel gas nozzle, although such an arrangement is not preferred. The invention works best, however, if a plurality of oxygen and fuel gas ports are provided, arranged in rows opposite each other as illustrated in FIGS. 2 and 10. If a plurality of preheat ports are used, the port spacing, dimension y in FIG. 2, should be uniform. Each oxygen port 18 should be directly opposite a fuel gas port 17. This preferred arrangement gives the most uniform and fastest preheat, however, non-uniform port spacing or staggered fuel gas and oxygen ports or both will also work.

The flame angle F, i.e. the angle formed by the axis of the flame 14 with respect to the surface of the workpiece W should be between 40° and 55°, for a standoff distance Z of 25 mm. If angle F exceeds 55°, the flame tends to gouge the workpiece. If angle F is less than 40°, the tip 27 of the high intensity zone 12 will be too far from the work surface T to give desirably short preheat times. The flame angle F is determined by the values of the parameters in Tables I and II. The above-enumerated preferred values of the variables will give a satisfactory flame angle, but those skilled in the art will recognize that many other successful combinations are possible.

The invention works best if the preheat fuel gas and oxygen ports are as close to each other as possible without the gases converging within the unit, thereby creating the possibility of premixing and flashback.

FIG. 4 illustrates the preferred location of starter puddle B with respect to the center line projection of axis 15 of scarfing oxygen nozzle 16, when making starts on the top surface T of a workpiece W. As illustrated in FIg. 4, the oxygen stream from slot 16 should impinge on the rear end C of starting puddle B with respect to the direction of the scarfing cut indicated by arrow J. This location of the starting puddle allows all the molten material from the puddle to be blown forward, thereby leaving none to form ridges or fins at the rear of the cut.

If a start on the end of the workpiece W is to be made, as shown in FIG. 5, then the results will be satisfactory if the scarfing oxygen stream from slot 16 impinges on any part of the starting puddle, since there is no work surface T to the rear of the puddle on which ridges may form and it does not matter if fins are formed on the end surface E.

FIGS. 7 through 10 illustrate alternative embodiments of the invention which, although not the preferred, are nevertheless capable of producing a stabilized post-mixed preheat flame.

FIG. 7 is side view of a scarfing unit that is similar to that shown in FIGS. 1, 2 and 3, except that the preheat oxygen and fuel gas ports, 18 and 17, respectively, are located in the lower preheat block 2. The apparatus functions similarly to the apparatus of FIGS. 1, 2 and 3.

FIGS. 8 and 9 show an arrangement in which the stabilizing oxygen is supplied from port 16' separate from scarfing oxygen slot 16. Here a stream of preheat oxygen 9 from port 18, impinges upon a stream of preheat fuel gas 10 from port 17 to form a post-mixed flame 14. The flame is stabilized by a low-intensity stream of oxygen 15' from port 16', directed proximate to the impingement 30, and in the general direction of the flame. The preheat and stabilizing ports 17, 18 and 16' are shown located in upper preheat block 1. They could also have been located in lower preheat block 2. After preheating is accomplished, a stream of scarfing oxygen from slot 16 is turned on to scarf the workpiece. As described previously, fuel gas discharged from port 19 helps sustain the scarfing reaction.

FIG. 10 is the same as FIG. 9, except that the stabilizing oxygen originates from an elongated, slot-like nozzle 16". The preheat oxygen and fuel could also be supplied from elongated, slot-like nozzles, although such an arrangement is not preferred.

EXAMPLES

Scarfing starts on the top surface of a workpiece, as shown in FIG. 4, were made, in the laboratory, using apparatus having the values set forth in Table I and the preferred values of Table II. The test results are graphically represented by curve X in FIG. 6, in which the initial temperature (T° C), of the workpiece is plotted along one axis, while the preheat time required (t) in seconds, is plotted along the other axis. For purposes of comparison, curve Y shows the results obtained under comparable conditions using the scarfing apparatus disclosed by Lytle in U.S. Pat. No. 3,752,460, while curve Z shows the results obtainable by a conventional post-mixed preheat flame formed by oxygen discharged from the scarfing nozzle and fuel jets, such as disclosed by Allmang in U.S. Pat. No. 3,231,431.

As illustrated by FIG. 6, for cold workpieces, the present invention is a significant improvement over prior art preheating methods, giving preheating times less than half that required by the Lytle method for workpieces above 200° C. For workpieces below 200° C, the present invention requires significantly less than half the preheat time of the Lytle method. Note that the graph indicates that trap oxygen method of Lytle is unable to achieve preheat times below 20 seconds for workpieces below 250° C, while the present invention requires less than 20 seconds to preheat a workpiece at 0° C. 

What is claimed is:
 1. A process for thermochemically scarfing a metal workpiece comprising:(1) preheating a spot on the surface of the workpiece where the scarfing reaction is to begin by directing a post-mixed preheating flame at said spot, said preheating flame being formed by:(a) discharging at least one stream of preheat oxidizing gas and at least one stream of preheat fuel gas from separate ports in such a manner that said streams impinge external to their discharge ports, above the work surface and in such manner that the axes of said streams form an acute included angle between them, and (b) stabilizing said preheating flame by discharging a low-intensity stream of oxidizing gas, the direction of said stabilizing stream being in the same general direction as the direction of said flame and proximate to the impingement of said preheat oxidizing gas and preheat fuel gas streams, and (c) continuing steps (a) and (b) until said spot reaches its oxidizing gas ignition temperature, and thereafter (2) directing a stream of scarfing oxidizing gas at an acute angle to the work surface at said preheated spot, while simultaneously (3) causing relative movement between said scarfing oxidizing gas stream and said work surface, thereby producing a scarfing cut.
 2. The process of claim 1 wherein said acute included angle is between 5° and 50°.
 3. The process of claim 2 wherein the oxidizing gas is oxygen.
 4. The process of claim 3 wherein said stabilizing oxidizing gas and said scarfing oxidizing gas are discharged from the same port.
 5. Scarfing apparatus comprising: (a) means for forming a post-mixed preheating flame, (b) means for discharging a stream of scarfing oxidizing gas through a scarfing nozzle, and (c) means for producing relative motion between the scarfing oxidizing gas and a workpiece, characterized in that said means for forming said preheating flame comprises:(1) orifice means for discharging a stream of preheat fuel gas, the axis of said orifice means being directed toward the workpiece to be scarfed, (2) orifice means for discharging a stream of preheat oxidizing gas, the axis of said orifice being directed to intersect the axis of said preheat fuel gas orifice means at an acute included angle, external to said orifices and above the surface of the workpiece, and (3) means for discharging a low-intensity stream of stabilizing oxidizing gas, said means comprising an orifice, the axis of which is directed to proximate to the intersection of the axes of said preheat fuel gas orifice and said preheat oxidizing gas orifice and directed in the same general direction as the resultant of said axes.
 6. The apparatus of claim 5 wherein said acute included angle is between 5° and 50°.
 7. The apparatus of claim 5 wherein said means for discharging stabilizing oxidizing gas and said means for discharging a stream of scarfing oxidizing gas comprise a common discharge port.
 8. The apparatus of claim 5 wherein said scarfing nozzle comprises a slot formed between an upper preheat block and a lower preheat block in spaced relation to one another.
 9. The apparatus of claim 8 wherein a plurality of orifice means for discharging a stream of preheat fuel gas are arranged in a row substantially parallel to a row of a plurality of orifice means for discharging a stream of preheat oxidizing gas.
 10. The apparatus of claim 9 wherein the row of orifice means for discharging a stream of preheat fuel gas and row of orifice means for discharging a stream of preheat oxidizing gas are located in the upper preheat block.
 11. The apparatus of claim 9 wherein the row of orifice means for discharging a stream of preheat fuel gas and row of orifice means for discharging a stream of preheat oxidizing gas are located in the lower preheat block.
 12. The process of claim 2 wherein the preheat fuel gas flow rate is from 1 to 3.5 SCMH per stream, the preheat oxidizing gas flow rate is from 1.5 to 6 SCMH per stream, the stabilizing oxidizing gas is discharged from a slot like nozzle, and the stabilizing oxidizing gas flow rate is from 3 to 10 SCMH per cm of slot width.
 13. Scarfing apparatus comprising: (a) means for forming a post-mixed preheating flame, and (b) means for discharging a stream of scarfing oxidizing gas through a scarfing nozzle, characterized in that said means for forming said preheating flame comprises:(1) orifice means for discharging a stream of preheat fuel gas, the axis of said orifice means being directed toward the workpiece to be scarfed, (2) orifice means for discharging a stream of preheat oxidizing gas, the axis of said orifice being directed to intersect the axis of said preheat fuel gas orifice means at an acute included angle, external to said orifices and above the surface of the workpiece, and (3) means for discharging a low-intensity stream of stabilizing oxidizing gas, said means comprising an orifice, the axis of which is directed proximate to the intersection of the axes of said preheat fuel gas orifice and said preheat oxidizing gas orifice and directed in the same general direction as the resultant of said axes.
 14. The apparatus of claim 13 where said acute included angle is between 5° and 50°.
 15. The apparatus of claim 13 wherein said means for discharging stabilizing oxidizing gas and said means for discharging a stream of scarfing oxidizing gas comprise a common discharge port.
 16. The apparatus of claim 13 wherein said scarfing nozzle comprises a slot formed between an upper preheat block and a lower preheat block in spaced relation to one another.
 17. The apparatus of claim 16 wherein a plurality of orifice means for discharging a stream of preheat fuel gas are arranged in a row substantially parallel to a row of a plurality of orifice means for discharging a stream of preheat oxidizing gas.
 18. The apparatus of claim 17 wherein the row of orifice means for discharging a stream of preheat fuel gas and row of orifice means for discharging a stream of preheat oxidizing gas are located in the upper preheat block.
 19. The apparatus of claim 17 wherein the row of orifice means for discharging a stream of preheat fuel gas and row of orifice means for discharging a stream of preheat oxidizing gas are located in the lower preheat block. 